Section 7: Load Rating New Bridges

Purpose

Metric #13 of
FHWA’s Metrics for the Oversight of the National Bridge Inspection Program
 covers Load Rating. This metric requires all bridges to be rated for their safe load carrying capacity for current conditions in accordance with the
AASHTO Manual for Bridge Evaluation (MBE)
 considering all State legal vehicles and routine permit loads. Provide load ratings in the design plans of all new bridges as described herein.

Load Rating Requirements

Load rate in accordance with the current
AASHTO Manual for Bridge Evaluation (MBE)
 with current interims. Use the Load and Resistance Factor Rating (LRFR) method outlined in Chapter 6 Part A of the MBE.
Use HL-93 live load as described in Article 3.6.1.2 unless design for a special vehicle is specified or warranted. Use the fatigue loading as described in Article 3.6.1.4 for fatigue limit states.
Use the limit states and associated load factors as indicated in Table 1 below in lieu of MBE Table 6A.4.2.2-1.
For new construction, use a condition factor of 1.0. Do not apply a system factor. If warranted, include any adjustment for ductility, redundancy, or operational classification as load modifiers on the force effects within the design.
For prestressed and post-tensioned concrete bridges at the service flexure limit state, assume an allowable tensile stress of 0.19 fc'
, regardless of exposure environment.
The following elements require load ratings:
  • Prestressed beams and girders for service flexure, ultimate flexure, and ultimate shear
  • Steel girders (straight and curved, I-shaped and tubs) for service flexure, ultimate flexure, and ultimate shear. Fracture critical steel girder systems require a fatigue load rating.
  • Post-tensioned concrete superstructures for service flexure, ultimate flexure, and ultimate shear
  • Reinforced concrete superstructures for ultimate flexure and ultimate shear
Table 5-1: TxDOT Limit States and Load Factors for Load Rating New Designs (Replaces MBE Table 6A.4.2.2-1)
Bridge Type
Limit State
Dead Load (1)
Design Live Load
Inventory
Operating
γDC
γDW
γLL
γLL
Prestressed Concrete
Strength I
1.25
1.50
1.75
1.35
Service III
1.00
1.00
0.80
N/A
Structural Steel
Strength I
1.25
1.50
1.75
1.35
Service II
1.00
1.00
1.30
1.00
Fatigue (2)
0.00
0.00
0.80
N/A
Reinforced Concrete (3)
Strength I
1.25
1.50
1.75
1.35
Post Tensioned Concrete (4)
Strength I
1.25 max, 0.9 min
1.50 max, 0.65 min
1.75
1.35
Service I (5)
1.00
1.00
1.00
N/A
Service III
1.00
1.00
0.80
N/A
  1. For most new structures, no dead load for wearing surface is employed in design and accounting for added future wearing surface is not required. Also, note that the entire concrete deck thickness is to be included in the DC load effects and the resisting capacity (include panels, no sacrificial thickness assumed).
  2. Only evaluate the fatigue limit state for fracture critical structures.
  3. Superstructure (eg. non-standard reinforced concrete slab or slab and girder (pan form spans)) and substructure (interior bent caps). Do not load rate bridge decks supported by girders, except where noted.
  4. Include the effects of creep, shrinkage, secondary effects (due to prestress/post-tensioning), uniform temperature change, and temperature gradient as indicated in Table 2.
  5. Applies to post-tensioned straddle bents and transverse analysis of post-tensioned concrete bridge decks.
    Table 2: Additional Loads and Load Factors for Post-Tensioned Concrete (6)
    Limit State
    1.25
    PS
    TU (7)
    TG
    Strength I
    1.25 max, 0.9 min (8)
    1.00
    0.5
    N/A
    Service I
    1.0
    1.0
    1.0
    N/A
    Service III
    1.0
    1.0
    1.0
    0.50
  6. Adapted from AASHTO LRFD Table 3.4.1-1 and Table 3.4.1-3.
  7. Only applicable in framed structures.
  8. Applies to segmental superstructures. For non-segmental superstructures use 1.0.
The following elements generally do not require load ratings, unless specifically needed based on the request of TxDOT or where the substructure might be a controlling element.
  • Reinforced concrete bent caps for ultimate flexure and ultimate shear
  • Reinforced concrete inverted tee bent caps for ultimate flexure, ultimate shear
  • Post-tensioned bent caps for service flexure, ultimate flexure, and ultimate shear
  • Steel bent caps (eg. integral caps, straddle box caps, etc) for service flexural, ultimate flexure, ultimate shear, and fatigue
Contact the Bridge Division for guidance on unique structures or elements not addressed above. For widenings, only provide load rating for the new superstructure construction, unless requested to load rate the existing. See Chapter 5 – Other Designs, Section 1 - Widenings for requirements on load rating of existing structure as suitable criteria for widening.
Do not load rate bridge decks with the exceptions of transversely post-tensioned decks of segmental box girders and bridge decks with girders spaced wider than 12 ft.
Determine the controlling load rating based on the range of limit states indicated in Table 1, identify the location (eg. girder, span, position) both in the supporting design calculations and the New Bridge Load Rating Summary for Design Load Form.

Standard Bridges:

For bridges developed from standards for superstructure, use the load rating information documented on the standard. If there is no load rating information in the bridge standards, assume an inventory rating factor of 1.0 and operating rating factor of 1.3, which are lower bound numbers. Standards modified for aspects that would not otherwise affect the load rating, may also utilize the load rating information documented on the standard. No supporting calculations are needed for such standard and modified standard bridges, but load rating information must be transcribed to the New Bridge Load Rating Summary for Design Load Form and indicated on the bridge layout.

Documentation of Load Rating:

Provide the load rating calculations with the archived Bridge Design Notes that are submitted as outlined in Chapter 6 – Design Notes and Calculations, Section 1 – Procedure for Archiving Design Notes. Submit the load ratings based on the component, limit state, and controlling location as indicated by the Bridge Load Rating Summary Report for New Design Form. Include the final controlling HL-93 Rating Factor (RF) at the Inventory and Operating Levels. Include the controlling inventory and operating load ratings on the bridge layout as indicated below and in the
TxDOT Bridge Detailing Guide.
HL-93 Loading:
Superstructure Inv/Opr Ratings= X.XX/Y.YY
Substructure Inv/Opr Ratings= X.XX/Y.YY OR “Substructure Not Rated”